driver core: platform: don't oops in platform_shutdown() on unbound devices

On shutdown the driver core calls the bus' shutdown callback also for
unbound devices. A driver's shutdown callback however is only called for
devices bound to this driver. Commit 9c30921f ("driver core:
platform: use bus_type functions") changed the platform bus from driver
callbacks to bus callbacks, so the shutdown function must be prepared to
be called without a driver. Add the corresponding check in the shutdown
function.

Fixes: 9c30921f

 ("driver core: platform: use bus_type functions")
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20201212235533.247537-1-dmitry.baryshkov@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>